As I read it (having just looked at several pages on the topic) you need to add a background
image, which is made up of a single block of colour.
So first get hold of the 'image' you intend to use and save it to somewhere convenient on your hard drive (such as My Pictures). You could either use something like Photoshop or just do a Google Images search, e.g. for blue
http://tinyurl.com/jadfzfd
or pink
http://tinyurl.com/jsqtvr7
Then go to 'Settings' in Google Calendar and add your background 'image'.